This invention relates to the removal of beads of photoresist at the edges of beveled wafers or substrates, and particularly to a method and apparatus removing such edge beads formed of positive photoresist.
Photoresists are used to form masks on wafers and other substrates used in photolithographic processes. The photoresist is typically applied to the substrate in an even coat of substantially uniform thickness by any of several well-known processes, including deposition and spreading the photoresist in a squeegee-like manner across the surface of the substrate. Selected regions of the photoresist are then exposed to light through a photographic mask to develop the exposed regions of the photoresist. The photographic mask is removed, and a base solvent is applied to the photoresist to wash away certain regions of the photoresist. More particularly, in the case of a positive photoresist, the exposed, or developed, portions of the photoresist are washed away with a solvent. Conversely, unexposed, or undeveloped, portions of negative photoresists are washed away with solvents. In either case, the remaining portions of the photoresist form photolithographic masks on the substrate so that additional material(s) may be applied to the exposed portions of the substrate. In some applications, the photoresist may remain in the structure being formed. In other cases, the mask formed by the photoresist may be removed with a solvent, often carrying with it any unwanted materials on the photoresist that were deposited onto the substrate and photoresist during processes subsequent to the formation of the photoresist mask.
One problem in connection with fabrication of substrates employing photoresists resides in the formation of edge beads along the edges, including the back and front edges, of the substrate. Where the substrate has a beveled angle at the edge, the thickness of the photoresist in a direction normal to the top surface of the substrate is greater at the edge than in other regions. The thicker edge beads may create problems during subsequent processing of the substrate. Consequently, it is desirable to remove the photoresist edge bead from the edges of these substrates.
Traditionally, a solvent was employed to remove the edge bead, but due to the varying thickness of the edge bead, the amount of solvent employed could not be accurately controlled, thereby leading to ineffective removal of the edge bead photoresist, or damage to the substrate, or both. Positive photoresists presented the additional problem that because the edge bead consisted of largely unexposed photoresist, a strong solvent such as Ethyl Lactate, was required to dissolve the excess photoresist. However, this solvent often created defects at the beveled edge of the substrate. Moreover, the uncontrolled thickness of the edge bead in the processing direction normal to the top surface of the substrate often resulted in inadequate removal of the edge bead.
Edge exposure systems have been used to expose photoresist edge beads from substrates so that the beads can be removed by the develop solvent. However, these edge exposure systems exposed the bead from the top or working surface of the substrate where the edge bead is thickest, leaving unexposed photoresist on the edges of the substrate. Consequently, the solvent was ineffective to remove the edge bead.
The present invention is directed to removal of positive photoresist edge beads by directing light to the edge beads at an angle to the substrate to thereby more effectively expose the positive photoresist edge beads, allowing ready removal.
In accordance with one aspect of the present invention, a light source is provided to develop the photoresist edge bead. Apparatus directs a beam of light from the light source to the photoresist bead along a plurality of non-parallel paths approximately normal to an irregular surface of the photoresist bead.
In some forms of the invention, the apparatus directing light includes a light guide that directing light from the light source to the edge of the substrate along the plurality of paths. In various embodiments, the light guide includes light reflectors, light transmitters, lenses, and fiber optic cables. In various other embodiments, the light guide is supported on a mount that moves the light guide to various positions relative to the edge bead to direct light along a respective path for each position.
In other forms of the invention, a plurality of light sources may be employed to direct exposing light from various angles to the edge bead.
In accordance with another aspect of the invention, a positive photoresist bead is removed from an edge surface of a substrate. The photoresist bead is exposed with light along a plurality of non-parallel paths approximately normal to the surface of the photoresist bead. The exposed photoresist bead is then removed, such as with a solvent or the like.